Introduction to Shell Scripting

Introduction link

Bourne-again shell is the free command interpreter of the GNU project. Its name is a play on words (Bourne again / born again) referring to the historic Unix shell, the Bourne shell. Based on the latter, it brings many improvements, notably from the Korn shell and the C shell.

The original author is Brian Fox of the Free Software Foundation, later succeeded by Chet Ramey. The original Bourne shell was written by Steve Bourne.

Bash is free software published under the GPL license. It is the default interpreter on many free Unix systems, particularly on GNU/Linux systems. It’s also the default shell for Mac OS X and has been ported to Windows through the Cygwin project.

Bash uses the readline library which allows it, like the C shell, to automatically complete command and file names when typing the TAB key, which considerably speeds up work. The UP and DOWN keys allow easy navigation through command history.

Creating a Shell Script link

Creating a script is very simple, just create a file and make it executable:

  touch test.sh && chmod 700 test.sh
  

Data Streams link

Shell data streams are transported through three different channels:

• standard input
• standard output
• standard error output

Standard Input link

Standard input is the input data channel used by the system. By default, it’s the keyboard.

Thus, Shell commands take their parameters from the standard input.

Standard Output link

Standard output is the data output channel. This is the channel through which the data resulting from the execution of a command flows. It’s usually a terminal, meaning the screen.

Thus, Shell commands very often write results to standard output.

Standard Error Output link

Standard error output is the channel through which error messages pass, usually the screen. Sometimes a window is specially dedicated to this channel.

Whenever an error code is generated by a command, a message is sent on this channel.

Redirections link

It’s possible to temporarily change the standard inputs and outputs during the execution of a command.

For example, you want to write to a file the list of files in a directory. The ls command allows you to list the files in a directory. This command sends the result of its search to standard output (screen).

• Example:

  $ ls
amoi.c           montage.jpg     tp3.c
lettre.doc       tp1.c           zizitop.mp3
monprog.c        tp2.c
  

To redirect standard output to a file, use the special character >.

• Example:

  $ ls > liste.txt
  

If you display the contents of the file on the screen, you’ll see that it contains what the command should have displayed on the screen.

• Example:

  $ cat liste.txt
amoi.c           montage.jpg     tp3.c
lettre.doc       tp1.c           zizitop.mp3
monprog.c        tp2.c
  

The > character allows you to create the file if it doesn’t exist when the command is executed. If the file already exists, its content is overwritten.

To keep the file’s content intact and write to the end of it, use the special character ».

• Example:

  echo "List of my directory" >> liste.txt
  

The echo command allows you to display text on standard output, which is here redirected to the liste.txt file, to which the character string passed as an argument is written.

• Example:

  $ cat liste.txt
amoi.c           montage.jpg     tp3.c
lettre.doc       tp1.c           zizitop.mp3
monprog.c        tp2.c
List of my directory
  

We see that the file’s content hasn’t been overwritten and that it contains an additional phrase.

The following table summarizes the special redirection characters.

Special Characters link

In addition to the standard data stream redirection characters, the Shell has characters whose meaning is very… special. Here they are grouped in the following table.

Shell Variables link

Environment Variables link

The Shell, like MS-DOS, has environment variables that allow keeping important information in memory such as the user’s login (stored in the $USER variable) as well as their home directory ($HOME), the list of directories in which to look for executables of external commands ($PATH), and many others…

The env command displays the list of all the Shell’s environment variables with their values.

You can display available variables with the env command.

Other Variables link

Declaration link

The user can easily declare new variables by directly assigning a value.

Syntax: name=value

The variable’s value can be numeric or a character string, its format doesn’t matter.

Example: EMAIL=xxx@mycompany.com Manipulation

A variable can be used in any circumstance as long as it’s in the Shell. Its name must be preceded by the dollar sign ($) and be in braces ({}) if another word is contiguous to it.

Examples:

  $ echo $EMAIL
xxx@mycompany.com
  

  $ echo "My email: $EMAIL"
My email: xxx@mycompany.com
  

  $ echo 'My email: $EMAIL'
My email: $EMAIL
  

In the first example, the EMAIL variable is passed directly as a parameter to the echo command which displays it after the Shell has evaluated its value.

In the second example, a string containing the same variable is passed as a parameter to echo. Double quotes are special characters ("…") that force the Shell to evaluate the value of variables contained in the string between double quotes. The echo command therefore displays the string on the screen.

And finally, this time the single quotes forbid the Shell from evaluating the value of the variable. So echo displays the raw string.

Other Examples link

  $ moi=deimos; echo $moi
deimos
  

  $ phrase="Hello $moi"; echo $phrase
Hello deimos
  

  $ rep=`pwd`; echo $rep
/home/deimos/data
  

In the second example, the variable is in braces so that the Shell can distinguish it from the characters that follow.

In the last example, the rep variable contains a string between grave accents, which forces the Shell to interpret it as a command and replace it with the string that would be returned on standard output when executing said command. Command Files

Command files (scripts) are text files that contain Shell commands ordered by control structures. Execution

To execute a script, several solutions:

• Syntax: sh script

This syntax launches a new Shell process that reads its commands from the script file. This file must be readable.

• Syntax: . script

The terminal is momentarily replaced by the script file that must be readable (no new process created).

• Syntax: script

Launches a new Shell process that reads its commands from the file which must be readable and executable.

• Syntax: exec script

The current Shell is replaced by a Shell process that reads its commands from the file which must be readable and executable.

Parameters link

It’s possible to execute a script by passing arguments to it like any other command.

The following table summarizes the variables accessible to a script:

Creating Variables on the Fly link

Since bash is not a real programming language, there are many things that become difficult to do when you push it to its limits. In short, if you need to declare variables on the fly, here’s an example:

  f=0
for j in `echo $nodes` ; do
   f=`expr $f + 1`
   local porttest_node$f=`ssh root@$node1 netstat -auntp | grep :$clientport | grep -c "LISTEN"`
done
  

Reading a File Line by Line link

If you need to read a file line by line, as you can do in some more advanced languages, here’s an example:

  while read line; do echo "$(date),$(hostname),$line"; done < somefile.txt
  

Capturing Signals link

Capturing signals is so practical. For those who don’t understand or have no notion of these mechanisms, it’s quite simple. During a SIGHUP and other program closures, you can perform various actions using the trap command:

  trap "echo \"$0 process $$ killed on $(date).\"; exit " HUP INT QUIT ABRT TERM STOP
  

Hiding a Password During Keyboard Input link

It may be useful when a user enters something on the keyboard to hide it from the screen. This is done as follows in shell script:

  save_state=$(stty -g)
echo -n "Password: "
stty -echo
read password
stty "$save_state"
echo ""
echo "You inserted $password as password"
  

References link

Documentation on Unix initiation and shell scripts